Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (“CMOS”) image sensors (“CIS”), has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
FIG. 1A illustrates a conventional front side illuminated CIS 100. The front side of CIS 100 is the side of substrate 105 upon which the pixel circuitry is disposed and over which metal stack 110 for redistributing signals is formed. The metal layers (e.g., metal layer M1 and M2) are patterned in such a manner as to create an optical passage through which light incident on the front side CIS 100 can reach the photosensitive or photodiode (“PD”) region 115. To implement a color CIS, the front side further includes a color filter layer 120 disposed under a microlens 125. Microlens 125 aids in focusing the light onto PD region 115.
CIS 100 includes pixel circuitry 130 disposed adjacent to PD region 115. Pixel circuitry 130 provides a variety of functionality for regular operation of CIS 100. For example, pixel circuitry 130 may include circuitry to commence acquisition of an image charge within PD region 115, to reset the image charge accumulated within PD region 115 to ready CIS 100 for the next image, or to transfer out the image data acquired by CIS 100.
Crosstalk is a serious problem in image sensors. There are three components of crosstalk: a) electrical crosstalk, b) optical crosstalk, and c) color crosstalk. Electrical cross talk is caused by the drifting of charge carriers generated deep in the semiconductor epitaxial layers from their site of generation into neighboring pixels. Optical crosstalk is caused by the diffraction and/or scattering of light off of metal lines and at interfaces between the backend dielectric layers. A major source of optical crosstalk is light incident normal to the flat regions between the microlenses, which hit the metal lines directly underneath and scatter off to neighboring pixels (illustrated in FIGS. 1B and 1C). FIG. 1B is a plan view of eight neighboring CIS 100 within a color filter array 140. The pixels or CIS 100 are arranged within color filter array 140 using a Bayer filter mosaic (e.g., RGBG or GRGB). As illustrated in FIG. 1B, microlenses 125 of each CIS 100 are separated by gaps 145 between the pixels. FIG. 1C illustrates how light incident on gaps 145 scatters off of the metal layers within metal stack 110 into adjacent pixels.
Color crosstalk results from the finite (nonzero) transmittance of color filter 120 to wavelengths outside its target pass band, such as the finite transmittance of green and blue wavelengths through a red filter. To address color crosstalk the thickness of color filter 120 is chosen to maximize the transmittance within its pass band while greatly attenuating outside this range. The resulting color crosstalk may still be unacceptably high for applications which require high color fidelity. One solution is to increase the thickness of color filter 120. However, increasing the thickness also increases the absorption in the pass band, thus reducing the net quantum efficiency. Another disadvantage is the resulting increased backend height, which places microlens 125 further away from PD region 115, further increasing optical crosstalk. Accordingly, a limitation is imposed on thickness of color filter 120.